In a logging system, the logging cable customarily encloses a plurality of conductors. A common cable arrangement is deployment of seven conductors in an armored logging cable which has a strength member suitable for supporting the cable at great depths in a well borehole. Indeed, the multi-conductor cable is normally wrapped around a spool or drum to store upwards of 25,000 feet of cable to thereby enable suspension of a logging tool in the well subjected to logging. Ordinarily, this involves a logging tool which is lowered rapidly into the deep well at various stages of drilling, usually in open hole, and usually exposing the cable to the elevated temperatures found at the bottom on the well. The circumstances and conditions in which the cable is used creates problems as will be described.
The logging tool incorporates multiple logging devices. One of the logging devices in common use is a resistivity log. Another logging device is an electrode which provides the spontaneous potential of the formation adjacent the logging tool, or in other words, an SP measurement. As will be understood, SP measurements are relatively small, less than one volt and typically just a fraction of a volt. The logging cable will support a sonde which incorporates one or more tools in the sonde. A resistivity logging tool requires substantial electric current to create fields near the sonde which extend into the adjacent formations to controlled depths, thereby measuring formation resistivity. In the logging cable, the currents can therefore be substantial to power this equipment. Assume for purposes of illustration that the logging cable is 25,000 feet in length, and further assume that it is used at the surface on a winter day when the logging cable at the surface has a temperature of approximately 0.degree. F. As this cable is unspooled rapidly to lower the sonde into the well bore, the lower portions of the cable will be heated substantially, perhaps even as high as 300.degree. F. into hot formations. The rapid movement of the cable, the cable being deployed in the well from coiled storage on the storage drum and the rapid exposure of a portion of the cable to such high temperatures creates increasing noise in the cable. Moreover, assuming that an SP electrode is included in the logging tool, signals from the SP electrode are coupled up one of the individual conductors in the logging cable. As will be understood, this noise is coupled to the surface and may obliterate the SP signal on that particular dedicated conductor in the cable.
Some portion of this noise is derivative from noise which can be separately observed at the surface. The noise at the surface is in part dependent on the current which is supplied to the logging cable. Of course, this is an AC current because substantial AC power is required to operate the equipment in the sonde, and particularly to operate the resistivity logging equipment in the sonde. Accordingly, the surface currents provided to the cable from the power supply carry some indicia of the noise which will be observed on the SP signal. Indeed, this noise signal is in some measure impressed on the conductor dedicated to the SP signal. Sad to say, a direct proportional relationship between the noise measured at the surface and the noise observed on the SP conductor is not readily discernible. In other words, simple direct subtraction of the surface noise signal from the noise laden output signal on the SP conductor is not sufficient. Rather, the signal on the SP conductor is in fact obscured with noise which is related to the surface observed noise from the power supply in a more complex fashion, and the present system provides an adaptive transversal filter for removal of noise in the signal. When implemented, the surface current flow from the power supply to the cable can then be observed and measured so that this signal, appropriately passed through the filtering system to be described, can then construct a constructed signal which is subtracted from the SP conductor signal for noise cancellation purposes. This provides a sufficiently pure SP signal.
The present disclosure is therefore directed to a system which furnishes an SP output signal after noise subtraction. This takes into account the variable coupling which occurs as a function of the variable current flow in the logging cable and the other factors which vary in use of the logging cable. The logging cable thus picks up the noise which is provided at the surface, subject to subtraction through the filtering system just described, and a reconstructed SP output signal is then formed.
In further detail, the present apparatus utilizes an AC ammeter at the surface connected to the surface power supply. That meter provides a signal to a current to voltage converter which then provides a signal to an analog to digital converter. In digitized form, a procession of measurements having the form of digital words is input to a serial transversal filter. That filter assists in forming an output signal which sums up the noise signal component. In addition to that, the logging cable has an SP output signal which is laden with noise. That signal is passed through an ADC and is converted into an output signal in digital form which is the sum of two signals components, one being the SP signal itself and the other is the noise portion added to it. That SP output signal is added to the signal output from the adaptive transversal filter so that the SP signal can be reconstructed and is then output with substantial noise cancellation.